Holographic data storage is expected as the most promising candidate for next generation optical data recording system, and various researches have been made. In these researches, holographic photopolymer materials take up their position as an attractive candidate for optical recording materials in write-once-read-many-times-data storage applications because they have a large refractive index contrast, a high recording sensitivity, a high resolution and stability in recording, and in addition, recording media for data storage applications can be constructed by easy processing (for example, refer to H. J. Coufal, D. Psaltis, and G. T. Sincerbox eds. “Holographic Data Storage (Springer Series in Optical Sciences, Vol. 76, p. 10, 2000)”, D. A. Waldman, R. T. Ingwall, P. K. Dal, M. G. Horner, E. S. Kolb, H.-Y. S. Li, R. A. Minns, and H. G. Schild, “Cationic ring-opening photopolymerization methods for volume hologram recording”, SPIE, vol. 2689, pp. 127-141, 1996, and L. Dhar, K. Curtis, M. Tackitt, M. Schilling, S. Campbell, W. Wilson, A. Hill, C. Boyd, N. Levinos, and A. Harris, “Holographic storage of multiple high-capacity digital data pages in thick photopolymer systems”, Opt. Lett., vol. 23, no. 21, pp. 1710-1712, 1998).
On the other hand, methods of angle-multiplex recording have been proposed, and definite results of hologram characteristics have been obtained. However, from the viewpoint of practical use, it is difficult to solve all the problems for improvement of optical recording density by these methods because of instability of recording due to oscillation.
In hologram data recording, in order to read out optical data stored by multiplex recording, it is necessary to record optical data by displacement of an amount of spatial shift distance or more, and a material for multiple recording of high density giving a small amount of spatial shift distance is desired.